Today's integrated circuit chips typically contain millions of transistors on a single chip. These transistors produce heat during operation of the circuit. In some cases, the heat generated from the transistors can increase the overall temperature of the chip to over a hundred degrees Celsius and adversely affect the proper functioning of the devices on the chip. With the speed of typical chips operating at hundreds of megahertz (MHz), one of the persistent problems facing integrated chip design is compensating for the effects of temperature on the performance of the devices embedded in the chip.
In an integrated circuit chip environment, a variation in temperature typically affects the entire chip area. For example, when temperature increases, all the devices in the integrated chip are generally subject to approximately the same temperature change. Moreover, changes in temperature affect the devices in similar manner because electrical conductivity is inversely proportional to temperature. Conversely, electrical resistance is directly proportional to temperature. That is, higher temperature usually slows down the speed of all devices in the integrated chip whereas lower temperature speeds up the devices.
In addition to the temperature variations, integrated circuits also suffer from process variations. The main cause of the process variation is non-uniform threshold voltages of transistors. During manufacturing process in particular, non-uniform doping concentrations may produce transistors that have varying threshold voltages ranging between 0.6 and 0.8 volts. Higher threshold voltages speeds up and improves conductivity of transistors while lower threshold voltages slows down and reduces conductivity of the transistors. The variation in the process therefore introduces variation in speed and conductivity of transistors in integrated circuits.
Traditionally, bias circuits have been designed and implemented in integrated chips to determine internal voltage and current levels over the operating conditions of circuits in the chips. In particular, bias circuits are generally designed to provide a stable operating voltages and currents to other circuits or devices in the integrated chip. Since the bias circuits define the operating voltage and/or current levels for in the circuits of the integrated chips, various conventional bias circuits have been designed to reduce the circuits' voltage and/or current dependence on temperature variation. For example, a conventional band-gap reference circuit provides an output bias voltage that reduces sensitivity to temperature variation. Often, the conventional band-gap reference circuit typically includes a high-gain operational amplifier in a feedback loop to improve temperature independence. However, the addition of an operational amplifier in a feedback loop increases circuit complexity which translates into a larger die area and added cost. Moreover, the band-gap reference circuit did not provide any means of compensating for variations in process.
Thus, what is needed is a bias circuit that can be implemented in an integrated circuit efficiently and at the same time compensate substantially for variations in temperature and process in an integrated circuit. The present invention satisfies these needs by providing an MOS bias circuit that counteracts the effects attributed to variations in temperature and process by using non-matching transistors.